In the past numerous optic and catoptric arrangements have been developed for channelling low level radiation to electro-optic sensors for detection purposes. Such devices have been used for detecting motion based on sensing changes in infrared radiation levels at intensities which are very near background levels. Typical catoptric detection devices used in the past are those described in U.S. Pat. Nos. 3,271,575 to Falbel; 3,413,468 to Astheimer; and 4,087,688 to Keller. By way of example, among the devices known in the prior art, there has been disclosed an internal reflecting truncated cone for maximizing the gain in infrared signals provided through the wide opening of the cone to a single immersed detector located beyond the truncated cone end. The maximum possible gain achievable with the internally reflective cone is only comparable to an f/0.5 optical system thus necessitating the positioning of a conical plug having a high refractive index between the truncated cone end and the sensor to further condense radiation which would otherwise be reflected back out the wide end of the cone.
Some of the prior catoptric devices reflect radiation from a continuous field of detection to the sensor. However, it has also been found advantageous to divide an optical field into an alternating pattern of detection zones and passive, i.e., dark zones, in order to more reliably detect minor fluctuations in radiation levels corresponding to the slightest motion of a body through the field. Keller discloses the use of a simple collecting lens in combination with a reflective tube for focusing direct and reflected radiation upon a radiation sensor. The combination, which has been useful in burglary alarm systems, divides an optical field into a plurality of isolated detection zones each separated by a dark zone. During movement of a radiation source through one or a number of the detection zones, the corresponding level of radiation transmitted to the sensor will vary significantly from the background level. In response, the electrical signal provided by the sensor to either a conventional pulse detector or an alternating current voltage detector will also fluctuate resulting in an alarm status.
Generally, motion detection devices known in the prior art are of a complex design, or require expensive lenses, or are physically large. Furthermore, such prior art systems have been difficult to provide adequate numbers of zones of coverage and to eliminate large dark areas. Often, focusing problems are experienced with lenses.